My new ignition system is composed of the following:

MSD 6al
MSD Pro Billet Distributor 8572
MSD Blaster coil
MSD 8.5mm Super Conductor wires

The distributor has the heavy silver springs installed for the time being until I get it set up correctly, then I will play with dialing in the mechanical advance. It also has the blue 21 degree bushing installed. That would mean with 10* initial advance, the total would be 31*. I am using a Holley Street Avenger carb, and I have the distributor's vacuum advance hooked up to the port identified in the Holley manual for this; it is on the choke side, above the throttle plates, and vacuum increases with throttle.Now for my question.

When I disconnect the vac advance and set initial to 10* and raise the rpm's, I get 31* total, as I should. As soon as I hook up the vac advance, the rpm's immediately raise about 500 rpm, and the timing totals over 40*....what is going on here? Without the vacuum advance, the idle speed and timing all appear normal.
Also, I am using an advance timing light, and the timing seems to jump around a little.....I seem to remember that only a regular timing light should be used with a MSD setup...is that right?

thanks in advance

 

I'm running the exact same MSD setup on my '72. MSD recommends using an non-advance timing light. I bought an Actron from Advance Auto (fairly cheap) and works fine. Charles

 

Mako72: how do you have yours set up? initial advance? what mechanical advance bushing? what springs? are you using vac advance? where do you have the vac advance hooked up...ported or full manifold vacuum?

 

anyone have any insight about this???

 

Mine did the same thing, though not anywhere near 40 degrees. Your vacuum advance provides additional advance at no load situations like idle and cruising. When you jump on it, the vacuum drops and the vacuum advance falls out. That's why you time it with the vacuum unit disconnected and the vacuum source plugged.

My car calls for 8 degrees BTDC as idle advance. I used the bushing for 28 degrees of additional advance and the springs that allow the advance in by 3200 rpm. This was a big change over stock where I only had about 20 degrees in at around 4,000 rpm.

I am running regular gas with no problems.

 

It's sound like you've got the vacum advance hooked up to to ported vacuum, which shouldn't provide any vacuum at idleif the carb is adjusted properly. I'd look to your idle adjutment of the carb. You may have the throttle blades open too far. allowing vacuum to the ported port at idle. Try backing off on the idle speed screw and bringing the idle speed back to where you want it with the idle mixture screws. Another option is to run the vacuum advance off the fulltime vacuum port (the lower one) and adjust your idle down. Unless you're running some of the fastburn type heads, 31º is low for total. Factory hipo engines used to run 36º total and 52º with vacuum advance.

 

7t2, I installed my new motor and associated MSD ignition a little over 4 years ago. It's been troublefree and I tend to forget it other than check the timing and throw in some plugs every once in awhile. I am running 10 degrees initial and 32 degrees total. I don't remember what springs and bushings I am running. As you know, MSD provides an excellent instructions on setting up your advance curve. In the quest to get the max from my engine, I recently requested some literature from Lars (The carb/ignition guru here on the forum). He graciously sent me his excellent papers on setting up distributor advance units and tuning the holley carb. I am currently running ported vac to my dist but Lar's paper has some interesting info on running full manifold vac. I just went thru my headlamp/wiperdoor vacuum system and repaired some major leaks. So I plan on going back thru my carb/ignition and reesetting everything.

 

You are hooking the vac adv to an unported source meaning a source that sees vac all the time, if you see high vac at idle you will see high vac adv when hooking the vac adv to a unported source. Unported sources are located below the throttle plates while ported sources are located above the throttle plates. A factory like set up has the vac adv hooked to a ported source so it sees adv only when you crack the throttle, sometimes people will run it using an unported source with a big cam to increase the adv at idle which will help smooth it out some.

 

Chevy High Performance has done an excellent article on the whole process.

http://www.chevyhiperformance.com/howto/97438/

 

Please help, I have a 1970 Vette with a sm block, and am considering getting the MSD ignition set up; can someone tell me the best location to install the MSD 6AL box?

Thanks.

 

I have had the same problem for the last month with the same set up. After $300 and a month later I was no better, new MSD 6A, new distributor (after rebuilding my original), carb (rebuilt my edelbrock plugs, wires, coil, rotor, wiring everything and I was going and broke at the same time. Here is what I finally did to get mine set up for manifold vacuum (not ported).

Slow your RPM down below 800 (to make sure your not getting and mechanical advance from the distributor weights. That is where my new distributor documentation said it would start).
Unhook and vacuum advance plug the carb and put a vacuum pump on the vacuum advance after setting your initial timing (my cam mfg. called for 14 degrees).
Now pull in your vacuum advance all the way with the pump to see what you have with your timing light (lower the idle screw if necessary to keep it under 800 RPM so your mechanical advance is not kicking in). I adjusted mine to 32 degrees (adjustable vacuum advance can on the dist). I also went back and checked to see a exactly what vacuum it took to get the vacuum to advance in all the way. Mine came out to be 9-10 in. Hg. Now unplug the vacuum at the carb and see what vacuum you have there. It should be > 9-10 or your timing will be erratic because the vacuum to the advance won't be pulling it in all the way in. I have used a adjustable timing light with my MSD and have had no problems. This worked for me and now I have 52 degrees at 3000 RPM (14 initial, 32 initial+vacuum, and 52 total). Sorry to be so long winded, but after this past month of going through the whole thing, I feel like I almost know what I'm doing.

YBnormal...drive a vette

 

Next time stick with the GM HEI, should get over 100,000 miles of trouble free driving

 

Got mine attached to the passenger side wheel well. No overflow tank there on my '72.

 

In case you haven't seen it, here is Lar's paper on the three types of vacuum and how they are used.

http://home.comcast.net/~chadwick.ro..._explained.pdf

 

We picked up a 6a box and blaster coil but haven't used it yet. I would have preferred to use just a MSD module and run the stock coil but I was voted against on that idea!
I was at the Englishtown NJ Corvette show & Go on Saturday and conducted an informal survey on the MSD boxes. Most guys are running the 6Al with limiter. approx 1/2 of them had a box fail within 3-7 years, others had longer life and others are still running the orignal box they put in. Most were mounted in the engine bay,either on the right or left inner fender. I did see one mounted in front of the radiator which would give it good air flow. I'm still not sold on them but it looks like we're going to give it a shot and see.
Gary